The present invention relates to an ink jet recording apparatus which ejects ink under vibration from a nozzle, selectively charges ink drops by means of a charging electrode in a position where the ink separates into a drop, and deflects charged ink drops by means of a deflection electrode toward predetermined positions on a sheet of paper.
In an ink jet printing apparatus of the type described, ink continuously ejected from a nozzle separates into a drop of a predetermined diameter during its flight. The separation of ink into a drop is effected by applying high frequency vibration of a constant period to the ink, which is communicated under pressure to an ink ejection head. Typical means for applying the vibration is a flat or annular electrostrictive vibrator. A string of ink drops are selectively charged in accordance with print data. When an ink drop moves through a predetermined deflecting electric field, its flight path is deflected depending upon the presence/absence of a charge thereon or a specific level of a charge if present. As a result, the ink drops individually impinge on predetermined positions on a recording medium or a gutter for collection, thereby printing out dots on the recording medium.
The charging principle in the art of ink jet recording utilizes the fact that when ink breaks into drops at a predetermined interval while being deposited with a charge at its tip due to electrostatic induction by an electric field developed by a charging electrode, the charge remains on the separated drops.
In this type of ink jet recording apparatus, the distance between an ink ejection nozzle and a paper is relatively long so that the ink is pressurized to a substantial level to allow an ink drop to fly as far as the paper along a stable path despite the charging and deflecting electric fields. Meanwhile, in order that ink drops may be formed regularly with a predetermined diameter and accurately follow expected deflection paths, stable and accurate controls have to be performed not only over the ink pressure but over ink viscosity, vibration pressure, charge amount, deflecting electric field, etc. Furthermore, ink drops cannot be charged properly or the deflection stabilized unless the application of a charge voltage (pulse) is accurately timed to the separation of a drop from the ink. To meet these requirements, it has been customary to control the ink pressure and/or ink viscosity to a predetermined stable value before a print charge control, and perform a phase search for predetermining the timing for applying voltage pulses, and a deflection adjustment for causing an ink drop charged at a given step to follow a path allocated thereto.
For the phase search, a contact or noncontact type charge detecting electrode is connected to a charge detector circuit whose major components are an amplifier, an integrator and a comparator. Charge voltage pulses having a narrow width are applied to the charge detecting electrode, while the phase of the charge voltage pulses is sequentially shifted at each preselected interval relative to the separation of ink into drops. When the charge detector circuit generates a signal indicative of "charged", a phase of the charge voltage pulses of that instant is determined as a proper charging phase. (This is followed by adjustment of deflection amount and printout operation.)
For the deflection adjustment, a contact or non-contact type charge detecting electrode is disposed one end of which is aligned with a certain deflection path. The charge voltage gain, ink pressure and/or deflection voltage is adjusted such that ink drops charged at a predetermined step of charge voltage are directed toward the end of the charge detecting electrode or barely miss it. For example, a flat charge detecting electrode may be positioned with its upper edge aligned with the flight path of the maximum deflection charged ink drops, as disclosed in Japanese Patent Application No. 55-48882/1982. In such a construction, the charge voltage amplification gain, ink pressure and/or deflection voltage is adjusted until ink drops charged in response to the maximum charge voltage code come to impinge on the upper end of the detector electrode. With this type of arrangement, if mechanical settings related to the flight of ink drops such as ink ejection direction, ink pressure and ink viscosity are controlled to desired values, the flight of drops can be adjusted essentially for all the deflection steps by aligning the flight of drops to a single deflection position.
However, the ink ejection axis (direction of flight of a straightforward ink drop) may become deviated from a desired value. Then, even if one point (e.g. maximum deflection print position) is adjusted into alignment with desired one, other points, particularly minimum deflection print position and low deflection print positions adjacent thereto, will still remain deviated from desired ones. In the case of a single nozzle type apparatus, although such a deviation causes disturbance to a reproduced image, the disturbance is relatively insignificant. However, such disturbance is noticeable in the case of color recording or divisional recording for superposed or divisional recording which uses jets of ink ejected from a plurality of nozzles, because the deviation in print position differs from one nozzle to another.